The present invention relates to a superimposing control method for a machine tool, in which a plurality of control systems are controlled by a computerized numerical control (CNC) device.
There is already known a superimposing control method in which motion of an arbitrary axis in one control system is superimposed on motion of an axis in another control system, to thereby control the axis of the latter control system, in a machine tool controlled by a numerical control device having multiple control systems.
FIG. 2 (PRIOR ART) is a block diagram showing the principle of a conventional superimposing control for a lathe having two control systems. Two control systems are provided with each having feed shafts for X and Z axes and such that each are controlled independently of each other. Interpolation processing for a motion command is performed based on a machining program for each control system, and after the motion command values to be distributed to X and Z axes are respectively subjected to acceleration/deceleration processing, they are output to respective servomotors for the axes, so that the X and Z axes of each of the control systems are controlled independently of each another.
There is a case where the control is performed with one control system as a reference system and the other control system as a superimposing system, so that the motion of an axis of the one system is superimposed on the motion of an axis of the other system. For example, as shown in FIG. 3, a workpiece W attached to a spindle 2 by a chuck 1 is moved in the Z-axis direction (right and left direction in FIG. 3) and the workpiece W is machined by a tool T1 moving in the X-axis direction (upward and downward in FIG. 3), as well as by a tool T2 movable in the X-axis direction and the Z-axis direction.
In the above case, the motion of the tool T2 in the Z-axis direction needs to follow the motion of the workpiece W in the Z-axis direction. Therefore, the control system for the tool T1 is set as the reference system and the control system for the tool T2 is set as the superimposing system, and the motion command for the Z axis of the reference system is added to and superimposed on the motion command for the Z axis of the superimposing system to control the Z axis of the superimposing system. Since the motion of the workpiece W in the Z-axis direction, i.e. the motion in the Z-axis direction of the reference system, is superimposed on the motion of the tool T2 in the Z-axis direction, the tool T2 moves with the workpiece W and at the same time moves relatively to the workpiece W by an amount of the Z-axis motion command for the superimposing system. As a result the workpiece W can be machined by the tool T2 as commanded by the superimposing system. As a method for such superimposition, a value of the motion command for the Z axis of the reference system (a value distributed to the Z axis), which has been subjected to the acceleration/deceleration processing, is added to a value of the motion command for the Z axis of the superimposing system, which has been subjected to the acceleration/deceleration processing, as shown in FIG. 2, so that the servomotor for the Z axis of the superimposing system is controlled to be driven based on the superimposed value. In this case, the superimposing control is started and terminated in a state where both the reference system and the superimposing system are stopped.
This is because, if the superimposing control is started during the movement of the reference system, the superimposition value for the superimposed axis is not subjected to acceleration/deceleration processing and the motion command value for the reference axis is directly added to cause rapid acceleration of the superimposed axis, so that shock or vibration occurs or an alarm is issued due to an excessive increase of a position deviation. To explain this with reference to the example of FIG. 3, if the motion of the workpiece W is added to the Z-axis movement of the tool T2 of the superimposing system during the movement of the workpiece W, no acceleration/deceleration processing is performed on the superimposing motion command for the tool T2 and the velocity of the workpiece W is simply superimposed to cause rapid acceleration, so that shock or vibration occurs. Therefore, in the case where machining is to be carried out by the two control systems with a sequence of non-superimposing control, superimposing control and non-superimposing control, machining programs for the reference and superimposing systems are first individually executed in a non-superimposing mode, then executed in a superimposing mode after waiting until both motions of the two control systems are completed, and executed again in a non-superimposing mode after the respective machining programs for the superimposing mode are terminated.
However, each time the superimposing control is started or terminated, one of the two control systems has to wait until a machining program of the other control system is completed, to rise a problem that a machining time and thus a machining cycle are prolonged.
To solve the problem, a method may be employed in which the motion command value before subjection to the acceleration/deceleration processing for the reference system, is added to the motion command value before subjection to the acceleration/deceleration processing for the superimposing system, and then the obtained value is subjected to the acceleration/deceleration processing. This method is, however, disadvantageous in that a path error of the superimposed axis occurs unless the values of time constants of the acceleration/deceleration processing for the reference axis and the superimposed axis are the same or so close to each other within a certain range.